Wireless microphones in the frequency jungle
Nov 1, 2006 12:00 PM, BY SVEN BOETCHER
The set-up
The first step to set up a multichannel system is to check the frequency spectrum for available frequencies for wireless microphones. Ideally, the spectrum is scanned with a spectrum analyzer.
An alternative is to use a microphone receiver system that automatically finds available frequency slots. The selected frequencies need to be free from intermodulation. Calculating these frequencies requires a software application. Even in a mid-size system, billions of calculations are made to find intermodulation-free frequencies. Once the gaps in the spectrum are identified, then the frequencies can be calculated.
Filters are key
The problems of intermodulation can be eased by using receivers that are restricted to part of the UHF spectrum with steep fall-off bandpass filters. The performance is mainly determined by the quality of RF filters that are used. The purpose is to get rid of as many unwanted signals before the active part of the system.
Not only will the receivers have to operate near TV transmitters, but walkie-talkies, cell phones and other local devices should not block the receiver. The use of wireless in-ear monitoring adds to the potential for intermodulation interference. Great care, therefore, is required to produce highly selective filters providing the best rejection of unwanted signals to the receivers.
By taking these precautions, running a wireless microphone system in today's frequency environment is eminently possible. But the performance of a system and proper planning requires more attention to detail then ever before.
The worldwide approach to reduce the spectrum for broadcast and grant licenses in the UHF spectrum to new services will severely effect the operation of wireless microphones. A common approach needs to be found to allow the future use of radio microphones in areas of RF background.
Every technical approach must be taken to make the systems as smart as possible and allow coexistence with other signals. In the end, the allocation of bandwidth by government regulators is an obvious prerequisite to run a radio microphone system.
Finally, the David and Goliath of UHF are not enemies but companions, because where would a TV program be without wireless microphones? And what is a wireless microphone without its users in TV productions? The deployment of wireless microphone systems shows how spectrum can be shared with other users by careful product and system design.
Sven Boetcher is product manager for professional wireless microphones at Sennheiser Electronic, Wedemark, Germany.
Rules for wireless microphone use
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Place the antenna as high and near the scene as possible to get a direct line of sight between transmitter and receiver.
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Directional antennas can be used to suppress unwanted signals.
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Avoid any shielding of the transmitter antenna. For example, never bend the antenna between the body and the bodypack transmitter.
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Use low-loss cable between the antenna and receiver.
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Use a booster to compensate for cable losses.
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In a receiving system, put the booster directly behind the antenna in the RF cable.
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Do not overboost your system. For example, if you use only a short cable with 1dB or 2dB loss, a 10dB booster may overload the input stage of the receiver.
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Use only intermodulation-free frequencies calculated with special SIFM software.
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Never work in an occupied TV channel. This may reduce the working range and cause problems for TV reception.
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